This invention relates to apparatus and a method for the forming of elongated hollow metal sections into a predetermined shape or contour. It relates particularly to apparatus and a method for the bending or shaping of elongated hollow metal sections, such as aluminum extrusions, using "stretch forming" apparatus and methods. The stretch forming process for bending or shaping of an aluminum extrusion involves placing the ends of the extrusion into an opposed pair of jaws or clamps attached to a pair of opposed hydraulic cylinders and then applying sufficient tension through the hydraulic cylinders and jaws or clamps on the ends of the extrusion to "stretch" the metal in the extrusion beyond its yield point or elastic limit. While the metal is tensioned above the elastic limit, a forming die of desired shape and contour is pressed against the extrusion causing the extrusion to assume the desired shape and contour of the forming die. The tension on the ends of the extrusion is then reduced and the newly shaped extrusion is removed from the forming die and the stretch forming apparatus.
In the past, the stretch forming of elongated hollow metal sections, and especially thin walled aluminum extrusions, often produced crimps or wrinkles in certain portions of the walls of the section or extrusion as a result of the inability of the walls to resist the reshaping forces during the stretch forming operation. Such crimps and wrinkles not only weakened the extrusion but also resulted in an extrusion of unacceptable appearance.
While in some cases the crimps and wrinkles could be eliminated by using a thicker walled section, such a solution added to the cost of the finished product and increased its weight. U.S. Pat. No. 4,803,878 issued Feb. 14, 1989 to Moroney not only discloses the above-described apparatus and process for "stretch forming" of elongated hollow metal sections or extrusions, but also discloses one proposed solution to eliminate the crimps and wrinkles formed in the reshaping of thin walled extrusions. Moroney suggests that the crimps and wrinkles can be reduced or eliminated by introducing a gas under pressure into the interior of the elongated hollow metal section or extrusion while it is being stretch formed. Moroney claims that the internal gas pressure is sufficient to support the internal walls of the extrusion during the stretch forming operation and will prevent the formation of crimps and wrinkles. While the use of an internal pressurized gas has helped to reduce the formation of crimps and wrinkles, the use of the internal pressurized gas alone has not completely eliminated crimps and wrinkles in elongated hollow metal sections or extrusions of complex cross-sectional shape and has increased the reshaping cycle time and cost required to produce an acceptable finished product by the "stretch forming" process.
It has been known to use both external and internal mandrels to prevent the crimping and wrinkling of tubing, pipe and other hollow elongated metal sections while they are being bent. Some of the known mandrels are bendable or flexible to allow support by the mandrel throughout the bending operation. U.S. Pat. No. 3,747,394 to Cunningham discloses a flexible, expandable internal mandrel used to bend large diameter pipe. The mandrel is supported internally within the pipe on rollers. Cunningham's mandrel uses a plurality of pipe engaging shoes that are clamped tightly in place against the pipe by plurality of toggle joints.